1) Field of the Invention
The present invention relates to an outer casing for a flammable gas sensor.
2) Description of the Related Art
Conventionally, a catalytic combustion type gas sensor and a semiconductor type gas sensor are used as a gas sensor for detecting flammable gas, such as hydrogen and methane. Both types of the gas sensors include a heat source that is used for detecting the flammable gas. For example, the catalytic combustion type gas sensor includes a heater coil with a combustion catalyst as the heat source. The flammable gas causes combustion with the catalyst. The heat generated by the combustion causes a change in resistance of the heater coil, and the change is output as a change in voltage of the gas sensor. Thus, the flammable gas is detected.
On the other hand, the semiconductor type gas sensor includes a heater coil with a semiconductor layer. The semiconductor layer absorbs the flammable gas, and electrical conductivity thereof changes according to the absorption. The change is output as a change in voltage of the gas sensor. Thus, the flammable gas is detected.
The above gas sensors have a gas permeable cap to stabilize temperature equilibrium of the heater coil and to prevent explosion of the flammable gas. The gas permeable cap is made of wire gauze, sintered metal, porous ceramics, and the like. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open No. S55-82954.
The above gas sensors also have a mount base that is made of synthetic resin without gas permeability. The mount base supports a charging unit including pin electrodes, which pierce through the mount base and are electrically connected to a gas sensing element. The catalytic combustion type gas sensor also includes a compensating element that has the same temperature-equilibrium property as the gas sensing element. When the gas sensing element and the compensating element are arranged in a single housing, a heat shielding board that is made of metal or synthetic resin is arranged between the elements to avoid temperature interference, and to keep temperature and humidity uniform around the elements.
The gas permeable cap has a function of protecting the gas sensing element from various environmental factors. However, the gas permeable cap can decrease sensitivity of the gas sensor because gas permeability of the gas permeable cap is limited. The mount base does not contribute to the sensitivity because the mount base does not permeate a target gas inside the gas sensor.
Moreover, the heat shielding board, which is provided in the catalytic combustion type gas sensor to insulate heat between the gas sensing element and the compensating element, shields atmosphere between the elements inside the gas sensor. This is not preferable for stabilizing output voltage of the gas sensor according to temperature and humidity.
Furthermore, the gas permeable cap made of wire gauze, sintered metal, or conventional porous ceramics does not have resistance to a gas, such as an organic solvent gas, that can deteriorate the gas-detection function of the gas sensor. Therefore, additional equipment such as a supplemental filter of activated carbon is required to remove the gas. This increases a manufacturing cost of the gas sensor. Moreover, it is necessary to consider corrosion resistance if a metallic part is included in the gas permeable cap.